The present invention relates to scanning beam writing and reading systems and more particularly concerns improved methods and apparatus for employing a coded reference beam to define position of the write or read beam in the course of its scan.
In the writing of data by means of a flying spot scanner, an energy beam such as a laser beam, is reflected from a scanning mirror to scan a write medium along a linear scan path that is caused to move transversely relative to the scan path. The energy beam is modulated, generally being turned on or off in accordance with data to be written and, thus, writes a desired data pattern by means of a succession of spots or dots. In order to know when to turn the energy beam on and off in the course of each line scan it is necessary to know the location of the beam. This can be done by sensing initiation of each scan line if scan velocity is linear. However, many types of scanning devices and their optical systems exhibit a nonlinear scan velocity. Thus, in many systems, the speed with which the writing spot traverses the write medium varies in the course of a single scan line. Therefore, the location of the beam on the write medium is not a fixed nor readily compensible function of time. Also, in some scanning systems nonlinearities may be so great as to introduce unacceptable intensity variations in the course of the scan.
To account for such nonlinearities, systems have been devised to sense the actual location of the write beam. This is frequently accomplished by causing a reference beam to scan a periodic mask in synchronism with the scan of the write beam across the write medium. Beam energy transmitted by the periodic mask is then detected and employed to develop a clock signal used to control modulation of the write beam.
In such reference beam systems problems are encountered in the collection of energy transmitted from the periodic mask. A scan line may be 18 inches in length, in some systems, and thus, a mask of similar dimensions is employed. Collection of light over such a distance has required the use of fiber optics or a Fresnel lens. Other arrangements have employed elliptical reflectors for reflecting energy from the mask to a detector at one focal point of the ellipse, the scanner being positioned at the other focal point. Such systems are cumbersome, bulky and expensive.
Accordingly, it is an object of the present invention to provide a simple and compact reference system that minimizes or avoids the above-mentioned problems.